1. Field of the Invention
This invention relates to an improvement of multi-stage sprocket assembly for bicycle, the assembly including at least a small sprocket and a large sprocket, the large sprocket being formed with a plurality of specially designed teeth for assisting a shifting movement of a chain between the small sprocket and large sprocket.
2. Description of the Prior Art
In a multi-stage sprocket assembly of a bicycle which has a derailleur, to shift the chain from a small sprocket to a large sprocket, the large sprocket usually has to be specially structured to ensure a reliable shifting movement. U.S. Pat. No. 5,192,248 (Nagano) discloses a large sprocket embodiment which has a shift assist projection or a split tooth projection for engaging with an outer side face of a link plate of the chain so that the chain can be lifted upward for shifting movement. However such a structure costs higher. The split tooth also weakens tooth strength and has negative impact on safety. U.S. Pat. No. 5,413,534 (Nagano) teaches another sprocket structure which has chain support projections formed on the side surface of the large sprocket for picking up the chain radially outward during shifting movement. The large sprocket has to turn a large angle for shifting movement. As the chain is subject to heavy loading, the chain support projection easily gets worn out after long time of use, and thus will adversely affect shifting movement.
The two prior arts set forth above need the sprockets to turn a large angle and to raise the chain radially outward for shifting movement. FIGS. 1A and 1B illustrate the principle of such a shifting movement. S1 represents the small sprocket, S2 is the large sprocket, C is the chain, F is the driving direction, N is the sprocket position when the shifting movement starts, P is the shift assist projection or chain support projection disposed on the large sprocket. During shifting movement, the projection P raises the outer side surface of the link plate of the chain C radially outward while the inner face of the link plate is being suspended without support. The sprockets S1 and S2 turn a large angle .theta. for allowing the next outer side face of the link of the chain C to engage with the large sprocket S2 to complete the shifting movement.
As the chain C is structured alternately by a pair of inner link plates and a pair of outer link plates, there are two different shifting paths for engagement between the chain C and sprocket S1 and S2. The difference of these two paths is resulting from the interchange of positions of the inner link plate and the outer link plate. Conventional method of dealing with these two shifting paths is to allocate two sets of shifting teeth. When the first set of teeth does not achieve shifting movement successfully, the sprockets continuously rotate until the second set of the teeth are positioned properly to perform the shifting movement. The shifting efficiency is therefore dropped. Furthermore, conventional shifting movement structure mostly takes into account only for shifting from the small sprocket up to the large sprocket. How to shift from the large sprocket to the small sprocket is often being neglected, and thus causes undesirable shifting movement when shifting down is performed.